Listeria monocytogenes (LM) (a category 2 Biodefense pathogen) is being developed as a vaccine vehicle for combating infection as well as cancer. Such vaccines may have particular utility against mucosal pathogens as well as against tumors since robust CD4 and CD8 T cell responses are induced by LM infection. In addition, recombinant LM expressing heterologous proteins have potential as vaccines against a variety of systemic or mucosal infections by inducing other protective mechanisms, such as antibody responses. The overarching hypothesis to be tested in this proposal is that a complex interplay between regulatory and inflammatory signals will result in distinct cellular and humoral outcomes during an adaptive immune response in the intestinal mucosa that will affect immunity at other sites. Although much has been learned in recent years with regard to the T cell response to infection, particularly using systemically administered LM, little is known regarding the intestinal mucosal immune response to oral infection, the natural infection route for this and many other human pathogens. What studies have been done used wild-type (WT) LM, which is a human- derived strain, to infect mice. This is an issue since WT internalin A, the receptor that binds to intestinal epithelial cell E-cadherin (E-cad) to allow bacterial invasion, has high affiniy for human E-cad but binds poorly to mouse E-cad. This proposal utilizes a newly developed recombinant LM expressing a modified internalin A protein (InlAM rLM) that binds mouse E-cad with high affinity, thereby allowing infection of the intestinal epithelium and subsequent bacteria dissemination. Thus, the immune response to LM immunization can now be studied in a natural setting more closely paralleling human infection. We will use this system to determine the efficacy of InlAM rLM vaccination to protect against homologous and heterologous infections and to dissect the contribution of migrating and tissue resident memory T cells in mediating protection in the intestine or lung in three specific aims: Aim 1: Determine the functional outcome of LM vaccination. Aim 2: Determine the efficacy of oral LM vaccination in protection against homologous and heterologous infections. Aim 3: To determine the contribution of mucosal memory T cell subsets to secondary protection against challenge infections.